Gas produced by vaporizing liquid raw material such as metal organic compounds by means of a vaporizer has been often used as process gas used for film forming processes in semiconductor manufacturing apparatuses. An example of film forming process may include a process of using liquid raw material, for example, tetrakis(ethylmethylamino)zirconium (TEMAZ), tetrakis(ethylmethylamino)hafnium (TEMAH), strontium bis(tetramethyl-heptanedionate) (Sr(THD)2) or the like, to form a high-permittivity film on a semiconductor wafer (hereinafter abbreviated as “wafer”). In this system, a vacuum pump to vacuumize the interior of a reaction container (reaction tube) in which the film forming process is performed and a raw material tank which is a repository of liquid material are, for example, installed below a vaporizer or the reaction tube (at a height position of operation by an operator or in the underground) in order to make a foot print of the system as small as possible. A raw material supplying device including raw material supplying pipes, valves and so on supplies liquid material from the raw material tank into the vaporizer upward.
In some case, the vaporizer and/or the raw material tank may be removed for maintenance of the vaporizer and/or exchange of the raw material tank with a new one. In this case, if the liquid material contacts the air, toxic gas such as diethylamine (C2H5)2NH), dimethylamine (CH3)2NH) or the like may be generated or there is a possibility that the raw material supplying pipe is blocked by reaction products generated by reaction of the liquid material with water contained in the air. Accordingly, when the vaporizer or the raw material tank is removed from the system, the liquid material is removed in advance from the raw material supplying pipe. Specifically, for example, the liquid material is extruded upward against a force of gravity by purging inert gas such as nitrogen (N2) or the like from below (the side of the raw material tank) into the raw material supplying pipe and is discharged from a vent pipe provided in the front of the reaction tube (in the side of the raw material supplying pipe) through the raw material supplying pipe in the side of the vaporizer. Thereafter, for example, the interiors of the raw material supplying pipe and the vaporizer are cleaned by flowing the cleaning solution such as octane or the like from below through the raw material supplying pipe, the cleaning solution is discharged from the vent pipe by the inert gas, and then the interior of the raw material supplying pipe is vacuumized and dried by the vaporizer.
However, since the liquid material has a specific gravity higher (i.e., is heavier) than that of water or the like, when the liquid material is to be extruded downward by the gas, the liquid material stays in the raw material supplying pipe and is in a so-called bubbling state where bubbles are to rise in the liquid material. This makes it difficult to remove the liquid material from the raw material supplying pipe even with increase in a gas pressure. In addition, since such liquid material has a vapor pressure lower than that of water or the like, for example, the liquid material is hard to be volatilized even when the interior of the raw material supplying pipe is vacuumized before the cleaning solution is supplied. In addition, since the interior of the raw material supplying pipe is not opened, it is not possible to directly confirm whether or not the liquid material is completely removed from the raw material supplying pipe.
If any liquid material is left in the raw material supplying pipe when the cleaning solution is supplied into the raw material supplying pipe, there may be a case where some water contained in the cleaning solution reacts with the liquid material, which may result in attachment (adhesion) of reaction products on a surface of a diaphragm (valve body) of a valve disposed on the raw material supplying pipe, for example. This may require frequent exchange of the diaphragm or the valve, which results in increase in running costs of the system.
Accordingly, when the vaporizer and/or the raw material tank are removed from the system, the liquid material has to be removed from the raw material supplying pipe by continuously flowing gas through the raw material supplying gas for, for example, a few days in order to secure a period of time for liquid removal (a period of time for supply of gas) slightly longer than a period of time for discharging of the liquid material even if the amount of liquid material left in the raw material supplying pipe is only several tens cc. Accordingly, operation hours (film forming time) of the system are shortened as much as time required for the removal of liquid from the raw material supplying pipe, and cost of nitrogen gas increases. In addition, even when such gas is supplied into the raw material supplying pipe for a long time, some liquid material may still be left in the raw material supplying pipe. In addition, likewise, even the cleaning solution is hard to be discharged from the raw material supplying pipe since it is extruded by the gas directing from below to above. In addition, when the liquid material is to be returned to the raw material tank by the gas supplied into the raw material supplying pipe from above (the side of the vaporizer), there is a possibility of contamination of the liquid material in the raw material tank through the inner wall of the raw material supplying pipe.
There is no document addressing the above-described problems although some techniques regarding a raw material supplying pipe and a valve are arranged to flow liquid from above to below have been known.